Various organic electrolyte secondary batteries are known as batteries capable of achieving high capacities, high voltages and high energy densities. In each of the organic electrolyte secondary batteries (for example, lithium ion batteries), a porous polymeric film allowing ions to flow together with the electrolyte between the positive electrode and the negative electrode is installed as a separator between both the electrodes installed in opposite to each other.
In an organic electrolyte secondary battery, for example, in a lithium ion battery, unstable metals exist in the battery, and a short-circuit, ignition, etc. can be caused. Especially since molten lithium metal is highly reactive, it is necessary to break the circuit before the temperature in the battery reaches the melting point (=186° C.) of lithium for ensuring the safety. As a conventional countermeasure for it, it is widely known that a porous polyethylene film or polypropylene film with a thickness of about 25 μm having a melting point lower than that of lithium is used as a separator in the battery, in order that the separator can be melted to crush its pores for functioning as an insulator (shutdown property) before lithium is melted. For example, JP3-203160A discloses that a polyethylene film exhibits said shutdown property at a temperature lower than the melting point of lithium and proposes an explosion-proof secondary battery using said film.
However, the polyethylene film or polypropylene film used as such a separator has such problems that it is low in heat resistance, and that since there is a limit for thinning the film while ensuring the required strength, the significant enhancement of storage capacity cannot be expected considering the limited battery size. That is, if the film is merely thinned, portions locally insufficient in strength and portions insufficient in form integrity as a separator at high temperature may be formed, and such inconveniences as ignition may occur in the battery. In addition, a separator with desired ion permeability may not be able to be formed. So, the film cannot be made thinner than a certain level.
On the other hand, in recent years, as described in JP8-111238A, an electrolyte substantially free from flash point is proposed. Since the safety against ignition, etc. can be greatly enhanced if such an electrolyte is used there is a possibility that the separator can be thinned only if it can have any desired ion permeability.
Further, organic electrolyte secondary batteries are required to be further smaller in size, and organic electrolyte secondary batteries of the same size are required to be further higher in capacity and voltage, etc. So, the separators used in them are required to be further thinner.
Furthermore, in the case of separators used for the secondary batteries of hybrid electric vehicles and fuel cell vehicles, the temperatures in the engine rooms become high during running, and it is more advantageous to use the batteries at higher temperatures for obtaining higher output characteristics. So, the separators used in them are required to have further higher heat resistance.
For such an application, suitable are porous films made of aromatic polyamides (hereinafter called “aromatic polyamide porous films”), which have high stiffness, can be thinned, are substantially free from melting point, and have high heat resistance. With regard to the aromatic polyamide porous films, for example, production processes are disclosed in JP59-14494A, JP59-36939A, JP2001-98106A, U.S. Pat. No. 6,642,282, and JP2002-30176A.
Moreover, porous films are disclosed in Japanese Patent 2615976 and JP2002-293979A, and applications of aromatic polyamide porous films to battery separators are disclosed in JP11-250890A, JP2002-42767A and JP2001-43842A. That is, it is disclosed that the inventions described in Japanese Patent 2615976 and JP2002-293979 have been completed for the purpose of decreasing the change caused by temperature and humidity by controlling the expansion coefficient, that the inventions described in JP11-250890A and JP2002-42767A have been completed for the purpose of obtaining necessary ion permeability by controlling the pore size and the porosity, and that the invention described in JP2001-43842A has been completed for the purpose of improving the positive electrode oxidation resistance by forming an inorganic thin film on the surface.
However, said aromatic polyamide porous films are not satisfactory enough for such reasons that, for example, in the case where they are used for a long period of time at high temperature as in the engine rooms of hybrid electric vehicles, higher positive electrode oxidation resistance is needed, and that precipitated lithium metal clogs the pores.